Loss of a Riverbank: Knotweed and other invasives

By Jim Serio

The globalization of our planet has enabled the trade of goods and services between nearly all countries around the world. With the addition of worldwide trade, there have been an ever increasing number of non-native plants and animals tagging along for the ride, or even deliberately introduced into our environment. The Upper Delaware River, regrettably, has been the recipient of several of these invaders.

If you live or spend much time on the Upper Delaware River, you may have noticed that some sections of riverbank are now completely dominated by a tall bamboo-type plant. This plant is known as Japanese knotweed (Fallopia japonica). A native of Eastern Asia, Japanese knotweed was introduced in England in the 1800s and then brought to North America during the 1876 Centennial Exhibition in Philadelphia. The pretty plumes of white flowers and fast growing nature of this plant provided height and interesting screens in the backyard garden.

Knotweed stalks grow quickly in the spring to a height of about 10 feet. The wide leaves create a nearly complete canopy that shades the undergrowth. The effect on the riverbank is devastating. Native grasses have thousands of small roots that intertwine in the soil to create a stable riverbank that resists the erosive nature of the flowing river. Knotweed roots are large and create a lump or knot with exposed, unprotected soil all around. High spring flows wash all of that exposed topsoil down the river.

How much topsoil, you may ask? The average cubic foot of top soil weighs about 75 pounds. Assuming an average of six inches of top soil under the grasses of a healthy riverbank and a patch of knotweed that is 100 yards long by 12 feet wide, if the topsoil under that knotweed washes away, that is 135,000 pounds—or seven tons. A patch a mile long and 12 feet wide would lose 1,200 tons.

What does it all mean to those critters that live along the river? Flowing water is always changing and re-shaping the river channel. A stream channel is trying to become deeper and narrower. This is great for fish and, in particular, trout in the Upper Delaware. Deep, cool water is preferred by trout. Knotweed alters the natural progression of these changes. The loss of massive amounts of topsoil destabilizes the riverbank and tends to lead to wider and shallower river channels. A wider and shallower river results on much warmer summer water temperatures.

Knotweed is very difficult to eradicate, but it can be controlled by various methods including a late spring frost; cutting and disposing so cuttings can’t sprout; creating a greenhouse effect with plastic; and chemical control. A new method of chemical control is a system that injects concentrated herbicide directly into stems. This is initially labor intensive, but is proving to be effective in eliminating a patch of knotweed. This has the added advantage of not harming other plants in the area.

The bottom line is that you must be persistent and will probably need to employ several different types of control over a few seasons to completely eliminate this invasive pest.